The present invention relates to a cable, in particular an optical fibre cable, which is resistant to the radial permeation and to the longitudinal propagation of water.
The present invention also relates to a method for maintaining a high resistance to the passage of water, both in the liquid state and in the vapour state, inside cables, in particular optical fibre cables.
Cables, and in particular optical fibre cables, are used in ambient conditions which include contact with water, both in liquid form and in vapour form.
The presence of water molecules inside optical cables, and in particular close to optical fibres, results in reduction of the transmission capacities of the fibres.
The said reduction in the transmission capacities of the fibres is due in particular to the diffusion of water vapour across ink secondary and primary coatings on the optical fibre and subsequent condensation of water at the ink-secondary coating and glass-primary coating interface. This condensation can lead to local detachment between the ink and the secondary coating or between the glass and the primary coating, giving rise to irregular mechanical stresses (xe2x80x9cmicrobendingxe2x80x9d), which can cause attenuation of the signal transmitted.
Contact of optical fibres with liquid water can occur either following penetration of water from a poorly wrapped end (during storage or laying of the cable) or following accidental damage to this sheath.
The presence of water, in particular of water in the liquid state, and the possibility of its longitudinal propagation inside cables is also a possible cause of damage to the apparatus to which the cables are connected. In view of the above observations, it is therefore advantageous to block the propagation of water and to limit as much as possible the length of cable which, after contact with water, will have to be decontaminated.
Contact of the optical fibres inside a cable with water in the vapour state occurs when this water permeates through the layers which make up the optical cable, thus being able to get inside to where the optical fibres are located. Up to quite high relative humidity values (typically of about 75-80%), the optical fibres are not adversely affected by the presence of water vapour and can even remain under such conditions for years. Above this threshold, the high humidity in contact with the surface of the optical fibres can lead to drawbacks similar to those caused by contact with liquid water (for example delamination, local detachment between glass and coating and/or detachment between the various coating layers, microbending phenomena) which can result in increases in attenuation. Lastly, prolonged contact of water (either liquid water or water in the vapour state) with the surface of the fibre, such as that which occurs after glass-primary coating delamination, can lead to a reduction in the mechanical strength of the glass part of the fibre.
A range of solutions for limiting or preventing the entry of water into cables is disclosed in the prior art.
For example, to limit the penetration of liquid water into optical fibre cables, it is known practice to introduce a fluid blocking filling material, typically a grease or a thickened oil, into the structure of the cable in order to establish a physical barrier to the passage of water into the cable. These filling materials, since they not have any particular physicochemical interactions with the water, are also known as xe2x80x9cinert blocking materialsxe2x80x9d.
Examples of these inert blocking filling materials are disclosed in patents EP 811,864, U.S. Pat. No. 5,285,513, U.S. Pat. No. 5,187,763 and EP 541,007.
The introduction of the said inert blocking filling materials into the structure of the optical cable during production is often laborious, such as processing of the ends (xe2x80x9cheadsxe2x80x9d) of these cables, which need to be wrapped so as to prevent any loss of the filling material. In addition, during installation and/or maintenance of the cable, in order to be able to make junctions between the different pieces of cable, it is necessary to wash off the blocking filling material from all of the components of the optical cable, which can result in damage to the optical fibres due to the action of the solvents and friction.
Another known solution for limiting the ingress of water into optical cables envisages the use of water-swellable materials, i.e. substances capable of absorbing a certain amount of water, thereby increasing their volume. In contrast with the materials described above, these materials are also known as xe2x80x9cactive blocking materialsxe2x80x9d.
Typically, these water-swellable materials are distributed on supports made of fibrous plastic material, for example tapes or yarns, which are placed close to the cable structures which it is desired to protect against contact with water.
For example, U.S. Pat. No. 4,867,526 describes a cable comprising a tape made of nonwoven material (in particular polyester) impregnated with a solution of water-absorbing material, in particular polyacrylic acid made insoluble by crosslinking, which is capable of expanding on contact with water.
U.S. Pat. No. 5,138,685 describes a cable comprising a laminated tape consisting of two superimposed layers of nonwoven polymer material, between which is placed a water-swellable material in powder form.
U.S. Pat. No. 5,642,452 describes a cable comprising a yarn impregnated with water-swellable material, in particular polyacrylic acid. This yarn is wound around a central reinforcing element together with tubular elements containing the optical fibres which are filled with a conventional xe2x80x9cinertxe2x80x9d blocking material. According to the disclosure given in that patent, this configuration is capable of preventing the longitudinal passage of water in the star-shaped areas created by the helical winding of the tubular elements around the central element.
U.S. Pat. No. 4,767,184 describes an optical cable with a grooved core, in which grooves are placed several strips of superimposed optical fibres, each coated with a film of resin containing a water-swellable or swelling material. In combination with the strips of optical fibres with a coating containing water-swellable material, a coating of the same material applied to the grooved core can be used, whereas in the grooves in which no strips of optical fibres are present it is necessary to use a powder made of water-absorbing material.
German Patent DE 1,765,647, relating to conventional metal-conductor telecommunication cables, discloses a 4-wire cable wherein the cable core and the wires are lapped up with a 100 xcexcm thickness foil made of a low saponification highly polymerised polyvinylalcohol.
The Applicant has observed that if water-swellable fibrous tapes are used, during manufacture of the cable it is necessary to include an additional wrapping operation. Moreover, the problem of release of the water-swellable powders often arises, with the result, on the one hand, that the water-blocking effect is lower where it is needed and, on the other hand, during installation/maintenance, the structure of the cable needs to be free of the presence of these powders.
Moreover, once the water-absorbing effect is complete, the known water-swellable materials behave like inert filling materials, by establishing a simple physical barrier to the passage of water. It is thus necessary to provide a sufficient amount of these materials in the structure of the cable which it is desired to protect. However, in certain optical cable structures such as, for example, in plastic tubular elements containing optical fibres arranged loosely, the amount of material to be used is excessive and uneconomical and in these cases use is therefore made of conventional inert blocking materials. The possibility also exists that an undesired swelling of this water-swellable mass (even in the presence of small percentages of relative humidity) might give rise to attenuation phenomena of the signal transmitted, on account of the irregularly distributed pressures on the surface of the optical fibres, even under conditions which would otherwise not be harmful to the functioning of the cable.
In addition, as has been observed by the Applicant, optical fibres in contact with particulate material with particles larger than about 1 xcexcm in size may be subject to microbending phenomena, as a result of which the insertion of elements such as strips or yarns containing water-swellable powders in direct contact with the fibres presents appreciable risks of increasing the attenuation, even independently of the presence of moisture.
In the case of optical fibre strips coated with a film of water-swellable material, described in U.S. Pat. No. 4,767,184, the Applicant has observed that if the water-swellable materials used tend to swell, even to a very small extent, in the presence of relatively low percentages of relative humidity, which in themselves would not be harmful to the optical fibres, this swelling can result either in irregular distribution of the pressure on the fibre coated with this material, or to excessive ringing of the optical fibre, both of which being drawbacks which can result in attenuation phenomena of the signal transmitted.
The Applicant has also observed that after it has expanded, a swelling material is not capable of correctly filling all the interstices inside the cable, in particular in the case where the spaces to be filled are of highly irregular shape, such as, for example, the star-shaped areas created by the winding of optical tubular elements the spaces between the optical fibres and the like. In this way, the risk remains that in these spaces, even after the swelling material associated therewith has reached the maximum possible degree of expansion, channels remain in the structure of the cable, although of reduced size, through which channels water can run freely.
In addition, complete swelling of the water-swellable materials typically requires a certain amount of time, during which the flow of water is not completely blocked and this can lead to a considerable length of cable affected by longitudinal penetration of water, in particular in the case where the empty fraction of the cavity involved is large.
Finally, any of the above solutions practised in the prior art implies the insertion of an additional element (such as powders, tapes, foils etc.) into the cable structure. This generally results in introducing additional steps into the cable manufacturing process, as well as in possible cumbersome operations while connecting cable""s ends and/or repairing damaged portions of said cable.
According to the present invention, it has been found that a cable which has no elements of fluid or pulverulent type for blocking the flow of water can be made by combining the cavities present in this cable with a solid material capable of dissolving in the penetrating water to form, with this water, a solution which is sufficiently viscous to block or at least greatly impede this flow of water. In particular, said solid material can be advantageously formed into a structural element of the optical cable, such as a tube or a slotted core, apt to contain and protect optical fibres.
One aspect of the present invention thus relates to an optical fibre cable comprising a longitudinal cavity extending along the length of the cable, at least one optical fiber disposed inside said cavity and a solid and compact element associated with this cavity, characterized in that the said solid and compact element comprises a water-soluble polymer material which, following accidental ingress of water into the cable, is capable of forming an aqueous solution with a predetermined viscosity such as to block the flow of water within 10 meters of the point of ingress. Preferably, this solid and compact element comprises about 30% or more, preferably at least about 50%, of the said water-soluble polymer material. According to a particularly preferred embodiment, this solid and compact element is formed for more than about 75% of the said water-soluble polymer material.
Preferably, said aqueous solution has a viscosity of at least about 104 cP (centipoise) at 20xc2x0 C.
In a preferred embodiment, said solid and compact element is a structural element of the cable apt to contain and protect the at least one optical fibre disposed within said longitudinal cavity. According to a preferred embodiment, the said solid and compact element containing the at least one optical fibre is a tubular element and the said longitudinal cavity is defined by the inner volume of said tubular element. Preferably, said tubular element comprises a two-layer wall in which the inner layer is made from the said water-soluble solid material and the outer layer is made from a conventional water-insoluble polymer material. According to a further preferred embodiment, the said tubular element comprises a third outer layer made of water-soluble solid material. According to an alternative embodiment, the said tubular element is made from a single sheath of said water-soluble solid material.
According to an alternative embodiment, said structural element apt to contain the at least one optical fibre is a grooved core comprising at least one groove longitudinally disposed on the outer surface of said core and the longitudinal cavity is defined by the inner volume within said groove. According to an embodiment of the present invention, at least the walls of said groove are made from a water-soluble solid polymer material. According to an alternative embodiment, said grooved core is made completely from said water-soluble solid polymer material.
According to another alternative embodiment, the element made of water-soluble solid material included in a cable according to the present invention is a tape.
According to a further aspect, a cable according to the present invention comprises at least one sheath of polymer material arranged so as to form the said cavity and at least one solid element of water-soluble polymer material arranged inside this sheath, characterized in that in the presence of a relative humidity of greater than about 75% outside this cavity, the relative humidity inside this cavity is maintained below about 75% for a predetermined period of time. Preferably, the said predetermined period of time is at least twenty years.
A further preferred aspect of the present invention relates to a cable as defined above, characterized in that, in the presence of a relative humidity of substantially less than or equal to about 80%, the said water-soluble polymer material absorbs an amount of water of less than about 25% of the amount of water which can be absorbed by the water-soluble solid material under saturation conditions.
According to a preferred embodiment, the ratio between the cross section of the said cavity and the perimeter fraction of this cavity delimited by the abovementioned solid element made of water-soluble material is less than about 0.5 mm. Preferably, when the viscosity of the said aqueous solution is at least about 104 cP, the ratio between the cross section of the said cavity and the perimeter fraction of this cavity delimited by the said solid element made of water-soluble material is less than about 0.4 mm.
According to a further preferred embodiment, the cross section of the solid element made of water-soluble material is not less than 10% of the free cross section of the cavity with which the said element is associated, this cross section preferably being at least 20%, in particular up to about 40%, of the said free cross section.
According to a further preferred embodiment, a cable according to the present invention is characterized in that the water-soluble polymer material associated with the cavity of the said cable is also water-swellable. Preferably, this material swells by at least about 5% relative to the original volume, when placed in contact with water for about four minutes.
According to a preferred embodiment, the water-soluble polymer material of the solid element associated with the longitudinal cavity has a solubility in water of at least about 100 g/l. Preferably, the said material is capable of forming an aqueous solution with a viscosity of at least about 104 cP at 20xc2x0 C. Preferably, the said aqueous solution contains an amount of polymer material of between about 100 g/l and about 250 g/l.
Preferably, the said water-soluble polymer material contained in a cable according to the present invention has a breaking load of greater than about 15 Mpa and an elastic modulus of greater than about 100 Mpa.
According to a preferred embodiment, the said water-soluble polymer material contained in a cable according to the present invention is selected from the group consisting of polyacrylamide, modified polyvinyl alcohol, vinyl alcohol/vinyl acetate copolymers and polyvinylpyrrolidone, and mixtures thereof. Preferably, this material is a vinyl alcohol/vinyl acetate copolymer.
According to a particularly preferred embodiment, the said copolymer can be obtained by partial hydrolysis of the acetate groups of a polyvinyl acetate homopolymer. Preferably, the degree of hydrolysis of the acetate groups of the polyvinyl acetate homopolymer is between about 50% and about 95%, even more preferably between about 70% and about 90%.
Advantageously, the said vinyl alcohol/vinyl acetate copolymer has a viscosity index of greater than about 5. Preferably, the viscosity index of the copolymer is between about 8 and about 40, vinyl alcohol/vinyl acetate copolymers with a viscosity index of between about 10 and about 30 being most preferred.
For the purposes of the present invention, the expression solid, compact element is intended to refer to an element consisting of a material, or a mixture of materials, which, at the working temperatures of the cable (and in the near absence of water), is not fluid, fibrous or pulverulent, and has mechanical properties, such as elastic modulus, breaking load, elongation at break and the like, which are similar to those of conventional polymer materials employed to make the structural elements of the cable, such as, for example, cores, sheaths or tubular elements containing optical fibres. The term xe2x80x9cconventional materialsxe2x80x9d is referred in the present description to those material typically employed in the art for manufacturing the above structural elements and comprise within its meaning, although not being limited to, polymer materials such as polyolefins, for example polyethylene (high, medium and low density PE), polypropylene (PP) or ethylene-propylene copolymers (PEP), polybutylene terephthalate (PBT), polyvinylchloride (PVC) or polyamides (PA).
The expression xe2x80x9csolid element comprising more than about 75% of a water-soluble polymer materialxe2x80x9d is intended to mean that this solid element is made mainly of a water-soluble polymer material, with the optional addition of other minor components such as, for example, fillers, plasticizers, pigments, dyes, processing agents, biocides or stabilizers, present in an amount of less than about 25% by weight, preferably less than about 10%.
In the present description, the term water-blocking material, or water-blocking properties, is intended to refer typically to a material capable of blocking the longitudinal propagation of water inside the cable within a predetermined length of this cable. Preferably, this length is less than or equal to 10 meters.
The term xe2x80x9cwater-absorbing materialxe2x80x9d is intended to refer to a material which tends to absorb water from the surrounding environment.
In the present description, the term xe2x80x9cwater-swellablexe2x80x9d or xe2x80x9cswellingxe2x80x9d material is intended to refer to a water-absorbing material which, when placed in contact with water, increases in volume, after absorption of a given amount of water, although remaining in the solid state. This increase in volume depends on the type of material, on the contact time of this material with the water and on the amount of water absorbed. This definition includes materials which, on contact with water, show a volume increase of at least 5% relative to the original volume, up to an increase of more than 100% relative to the original volume for substances with particularly high water absorption.
The term xe2x80x9cwater-soluble materialxe2x80x9d is intended to mean that the water-blocking material used in a cable according to the present invention is capable of at least partially dissolving on contact with water, creating an aqueous solution with a predetermined viscosity value. In particular, the viscosity of the solution which forms will be such that it hinders the flow of the said solution in the cable. Preferably, this solution has a viscosity such that it essentially blocks a flow of water which has penetrated into a cavity, within a distance of less than about ten meters from the point of ingress of the said water.